What a cylindrical, a prismatic and a pouch cell each are
Nearly every electric car battery pack is built from cells shaped 1 of 3 ways. A cylindrical cell is a sealed metal tube, shaped like a large store bought battery. A prismatic cell is a flat, rigid case, like a thin metal box. A pouch cell has no rigid case, just a sealed foil pouch, so something else must clamp it in place inside the pack. All 3 hold the same lithium ion chemistry. Only the housing differs, and that housing decides how much a pack costs, how safe it is, and how much of the finished pack is actually battery.
What each shape costs, and why shape alone is not the reason
A cylindrical cell used in a production car today, the Panasonic 2170, costs 170 dollars per kilowatt hour, the unit that measures a battery cost against how much energy it stores. A prismatic cell, the Samsung SDI 120 Ah, costs 127 dollars per kilowatt hour. A pouch cell, the GM Ultium, costs 100 dollars per kilowatt hour, according to a peer reviewed comparison in the Latin American Journal of Energy Research. Those 3 cells also use 3 different chemistries, the material that stores the charge, not just 3 shapes. An independent cost model in Communications Engineering built the identical chemistry as both a cylindrical and a prismatic cell, changing only the shape, and found shape itself moves manufacturing cost by less than 1%. Most of the 70 dollar gap above is the chemistry each maker paired with its shape, not the shape itself.
These 3 cells also use 3 different chemistries, so this spread is not a pure shape effect. See the text beside it.
Show the numbers
| Cylindrical | 170 |
| Prismatic | 127 |
| Pouch | 100 |
How each shape holds up when something goes wrong
A peer reviewed comparison scored all 3 shapes for mechanical integrity, how well a cell resists being crushed, punctured or deformed, from 1, very weak, to 5, very good. Cylindrical scored 4. Prismatic scored 3. Pouch scored 1, the lowest, because it has no rigid case, only a foil pouch that a crash or internal pressure can puncture. A cylindrical metal tube also lets manufacturers fit built in safety hardware that limits or cuts off current if a cell overheats, hardware a prismatic or pouch case cannot hold, which the comparison found makes the cylindrical shape more secure against overheating. Because a pouch cell has no case, the abuse testing manual the United States Department of Energy uses, written by Sandia National Laboratories, requires pouch cells be physically constrained during crash testing, a rule it does not place on the other 2 shapes. A separate study built cylindrical and prismatic packs to the same voltage, capacity and discharge rate, and still found the cylindrical pack ran hotter at its peak, 310 to 322 kelvin, a temperature unit, against 304 to 315 kelvin for the prismatic pack.
Score is the comparison own 1 to 5 scale, from 1 very weak to 5 very good.
Show the numbers
| Cylindrical | 4 |
| Prismatic | 3 |
| Pouch | 1 |
How much of a pack is actually battery
Every finished pack also loses volume to housing, wiring and cooling hardware, so no shape turns all of a pack into stored energy. Only 28.0% of a cylindrical pack volume is actual cell, the lowest of the 3, against 40.2% for a prismatic pack and 34.3% for a pouch pack, measured in the same comparison. Part of the reason is plain geometry. Circles packed inside a rectangular container can never fill more than about 80% of it no matter how well engineered, a limit that does not apply to a flat cell, which can sit edge to edge against its neighbor with almost nothing wasted between them. No single shape wins on cost, safety and packing all at once.
Circles packed into a rectangular container can never fill more than about 80% of it, a geometry limit that partly explains why the cylindrical shape packs the least.
Show the numbers
| Cylindrical | 28.0 |
| Prismatic | 40.2 |
| Pouch | 34.3 |